Kevin is $4$ times as old as Daniel and is also $6$ years older than Daniel. How old is Kevin?
Answer: We can use the given information to write down two equations that describe the ages of Kevin and Daniel. Let Kevin's current age be $k$ and Daniel's current age be $d$. ${k = 4d}$ ${k = d + 6}$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $k$ is to solve the second equation for $d$ and substitute that value into the first equation. Solving our second equation for $d$, we get: ${d = k - 6}$. Substituting this into our first equation, we get the equation: ${k = 4}{(k - 6)}$ which combines the information about $k$ from both of our original equations. Simplifying the right side of this equation, we get: $k = 4k - 24$. Solving for $k$, we get: $3 k = 24$. $k = 8$.